1. Field of the Invention
The invention relates to a method and a device to estimate motion in a sequence of moving pictures.
Its applications include the making of digital television systems and the reduction of the data throughput in these systems.
2. Description of the Prior Art
According to known methods for estimating motion in a sequence of pictures, either the characteristic features of these pictures are made to correspond with one another or a differential estimating method is applied, using the spatial and temporal gradients of representative picture elements (pels or pixels). However, in the latter case, it is difficult to master the initializing of the gradient algorithms used. For example, according to a first known method, a few characteristic pels (angular elements, contours, etc.) are tracked temporally to estimate their motion and to initialize a gradient algorithm. However, in this case, before the tracking is done, it is absolutely necessary to resolve the problems of extracting angular picture elements and making them correspond with one another. This is a painstaking task and, as yet, gives very imperfect results.
According to a second known method, the mean square deviation of local variations in inter-picture (or possibly inter-frame) luminance are minimalized and the gradient algorithm is initialized by estimating the motion of the prior pel, i.e. the pel preceding the present pel on the same line. In this case, the convergence of the algorithm depends, to a great extent, on the initialization phase used, and this is generally done by taking the displacement to be that estimated for the pel preceding the present pel in the image scanning direction. However, to obtain valid results, the motion observed should be slow, there should be little variation in its characteristics between one pel and another in a scanned line, and it should be possible to favor one orientation and one direction of displacement. According to another alternative, the displacement estimated at the homologous pel in the prior image is still used. However, as above, this presupposes a small temporal variation in the motion and requires the use of a displacement image memory. Furthermore, problems arise in the zones where there are breaks (both spatial and temporal) in motion when these zones are formed by contours reflecting objects in motion.